This invention relates to the silicone gel compositions which possess a high resistance to poisoning, especially to poisoning by solder flux and by amine compounds.
A gel cured product of a silicone rubber (which will be called a silicone gel in the specification below) possesses a superior stability both in electric insulation capacity and in electrical characteristics and flexibility; therefore, such materials are being utilized for the potting and sealing of electric and electronic parts, particularly as a coating material by covering control circuit elements such as power transistors, ICs, and condensers, in order to protect them from thermal and mechanical damage.
Conventionally, various types of addition curing organopolysiloxane compositions are known to form silicone gels. For example, one composition already known comprises an organopolysiloxane having vinyl groups bound to silicon atoms and an organohydrogen polysiloxane having hydrogen atoms bound to silicon atoms, which results in a silicone gel through the crosslinking reaction in the presence of a platinum type catalyst. (Japanese patent application Kokai (laid open) 56-143241, JP application Kokai 62-39659, JP application Kokai 63-35655, and U.S. Pat. No. 4,771,119 which corresponds to JP application Kokai 63-33475, EP 254,153 and CA 1,296,830.)
However, a silicone gel is a cured product with a low crosslinking density and it tends to be affected more readily by catalytic poison than silicone rubbers which have a high crosslinking density. Recent environmental problems have promoted a non-cleansing trend for the solder flux which is utilized in the soldering of electric and electronic parts. As a result, problematic inhibition of the silicone gel curing inhibition has been observed, the problem being due to the amine compounds within the solder flux. Further, contact with epoxy resins, which are utilized in large amounts as potting materials for the electric and electronic parts, also causes curing inhibition due to the amine curing agent which is employed for curing of the epoxy resins, leading to the unsuccessful curing of the gel. The conventional silicone gel compositions described above can not respond to such problems.